Photochemical reaction device

ABSTRACT

The present invention provides: an oxidation reaction electrode that generates oxygen by oxidizing water; and a reduction reaction electrode that synthesizes a carbon compound by reducing carbon dioxide. The two electrodes are electrically connected. Also, the reduction reaction electrode ( 1 ) synthesizes a carbon compound by reducing carbon dioxide in a water-containing liquid using radiated light energy.

TECHNICAL FIELD

The present invention relates to a composite photoelectrode, aphotochemical reaction device, and a light energy storage device whichreduce carbon dioxide to synthesize a carbon compound using water as anelectron source.

BACKGROUND ART

In the related art, Non-Patent Document 1 describes an example of theuse of a photoelectrode in a two-electrode system in anoxidation-reduction reaction using water as an electron donor.

Non-Patent Document 1 discloses a technique in which a p-GaInP₂electrode is used as a reduction reaction photoelectrode, a WO₃electrode is used as an oxidation reaction photoelectrode, and light oftungsten-halogen lamp is irradiated in a 0.5 M potassium nitratesolution, to decompose water.

Non-Patent Document 2 discloses a technique in which a semiconductorcatalyst powder such as TiO₂ is suspended in water as a photocatalystfor reduction reaction which presents a reduction reaction of carbondioxide, and light is irradiated from an artificial light source such asa xenon lamp and a high-pressure mercury lamp while carbon dioxide issupplied, so that formaldehyde, formic acid, methane, methanol, or thelike is produced.

Patent Document 1 discloses a technique related to a method in whichlight is irradiated to a zirconium oxide semiconductor in water, andhydrogen and oxygen are efficiently produced from water using the lightenergy, and a method in which hydrogen, oxygen, and carbon monoxide canbe simultaneously produced from water and carbon dioxide using lightenergy under the presence of a catalyst for photodecomposition of water.

Patent Document 2 discloses a technique in which a semiconductorelectrode such as TiO₂ in a hydroxide salt solution and a gas diffusionelectrode which carries a Pd—Ru alloy catalyst in a hydrogen carbonatesolution are short-circuited, and light is irradiated on a side of thesemiconductor electrode, to reduce carbon dioxide on the side of the gasdiffusion electrode and produce formic acid.

Patent Document 3 discloses a technique in which light is irradiatedunder the presence of a cuprous oxide serving as avisible-light-responsive photocatalyst and triethanolamine serving as anelectron donor, to selectively produce methanol with a reaction in wateror formic acid with a reaction in acetonitorile.

Non-Patent Document 3 discloses a technique in which light is irradiatedonto a p-InP photoelectrode in a methanol solvent in which carbondioxide is dissolved under a high pressure (40 atm) and a constantcurrent of 50 mA is supplied so that carbon oxide is produced with acurrent efficiency of 89%.

Non-Patent Document 4 discloses a technique in which light is irradiatedonto a p-InP photoelectrode modified with lead in a methanol solvent inwhich carbon dioxide is dissolved, to produce formic acid with aFaradaic efficiency of 29.9% and light is irradiated onto a p-InPphotoelectrode modified with silver in the methanol solvent in whichcarbon dioxide is dissolved, to produce carbon monoxide with a Faradaicefficiency of 80.4%.

Patent Document 4 discloses a technique in which carbon dioxide isintroduced under a high pressure of 0.2-7.5 MPa to an organic solvent inwhich there are dissolved a photocatalyst selected from among metalcomplexes having a charge absorption band between a metal and a ligandin a ultraviolet region to the visible light region and an electrondonor selected from among organic amines, and light is irradiated underthis pressure to selectively reduce carbon dioxide to carbon monoxide.

Non-Patent Document 5 discloses a technique employing a catalyst inwhich a Ru complex is electrochemically polymerized on a carbon orplatinum electrode, in which an electrochemical bias of −0.8 V (vsAg/AgCl) is applied, so that formic acid is produced with a Faradaicefficiency of 85%.

Non-Patent Document 6 discloses a technique related to production ofhydrogen using Cu₂ZnSnS₄ (CZTS).

RELATED ART REFERENCES [Patent Document]

-   [Patent Document 1] Japanese Patent No. 2526396-   [Patent Document 2] JP H6-158374 A-   [Patent Document 3] JP H7-112945 A-   [Patent Document 4] Japanese Patent No. 3590837

[Non-Patent Document]

-   [Non-Patent Document 1] Turner et al., Journal of the    Electrochemical Society 155 (2008), F91-   [Non-Patent Document 2] Fujishima et al., Nature 277 (1979), 637-   [Non-Patent Document 3] Fujishima et al., The Journal of Physical    Chemistry 102 (1998), 9834-   [Non-Patent Document 4] Kaneco et al., Applied Catalysis B:    Environmental 64 (2006), 139-   [Non-Patent Document 5] Deronzier et al., The Journal of    Electroanalytical Chemistry 444 (1998), 253-   [Non-Patent Document 6] Domen et al., Applied Physics Express 3    (2010), 101202

DISCLOSURE OF INVENTION Technical Problem

In a reduction reaction of carbon dioxide using water as the electrondonor, a photoelectrode material having a high energy level of aconductive band is necessary for the reduction of carbon dioxide. When avisible-light-responsive photoelectrode having a short bandgap is used,the energy level of valance band is also necessarily high, and the useof water as the electron donor becomes difficult. Because of this, areaction cell of a two-electrode system in which two types ofphotoelectrodes are combined can be considered effective for efficientuse of the solar light over a wide wavelength.

In Non-Patent Document 1, hydrogen and oxygen production reactions bydecomposition of water are reported. However, Non-Patent Document 1 doesnot describe the reduction reaction of carbon dioxide at all. Thereduction reaction of carbon dioxide; for example, a reaction in whichformic acid is produced from carbon dioxide, has a standard electrodepotential of −0.196 V which is higher in energy than a potential (0 V)where hydrogen is produced from a proton. Therefore, on a surface of asingle semiconductor electrode such as a p-GalnP₂ electrode, a hydrogenproduction reaction having low energy is highly likely to occur, thereduction reaction of carbon dioxide is difficult to induce, andselectivity of the reduction reaction is low.

Non-Patent Document 2 is an example document which discloses a reductionreaction photoelectrode which presents a reduction reaction of carbondioxide. Non-Patent Document 2 discloses simultaneous production offormaldehyde, formic acid, methane, methanol, or the like. In addition,Patent Document 1 discloses examples in which only hydrogen is producedor hydrogen and carbon monoxide are simultaneously produced.

Production of hydrogen and simultaneous production of two or more typesof carbon dioxide reduction products by radiation of light arecharacteristics of an inorganic semiconductor photocatalyst, but, inconsideration of industrial usage, obtaining the product with a highselectivity is important. In Patent Document 2, 32% of the photocurrentproduced by radiation of light on titanium oxide is used for conversioninto formic acid, and the selectivity is low. In addition, only titaniumoxide, which is an ultraviolet-light-responsive semiconductor, is used,and there is a problem in that the usage efficiency of solar light islow. Moreover, although Patent Document 3 uses avisible-light-responsive photocatalyst, an expensive electron donor madeof an organic substance is required for promoting the oxidation reactionpairing with the reduction reaction of the carbon dioxide.

In Non-Patent Document 3, carbon dioxide is reduced using avisible-light-responsive photoelectrode, but carbon dioxide must bedissolved under a high pressure in order to improve reactivity. InNon-Patent Document 4 also, carbon dioxide is reduced using avisible-light-responsive photoelectrode, to produce formic acid andcarbon monoxide with high Faradaic efficiencies, but methanol must beused as a reaction solvent. In addition, a high bias voltage of −2.5V(vsAg/AgCl) is applied, and, thus, the advantage of reducing thereaction voltage using the photoelectrode material is not exploited.

In Patent Document 4, only an example using a rhenium complex is shown.It has been reported in academic papers or the like that, when a rheniumcomplex is used, carbon monoxide tends to be selectively produced. It isalso known that, when the rhenium complex is used, in order to realize aphotocatalytic reduction reaction of carbon dioxide, only light ofrelatively short wavelength of 450 nm or less among the visible light isused. In addition, although Patent Document 4 also shows a complex usingother metals, this document only lists possible metal elements, and theconfiguration with the complex using other metals is not realized. Inaddition, it is known in the research of dye sensitized solar cells orthe like that a ruthenium complex can absorb light of a longerwavelength, depending on its structure. However, in this case, thephotocatalytic chemical reaction does not occur, and, currently, onlyelectrochemical catalytic reaction through application of electricity isrealized.

A reason why the reaction product selectivity on the semiconductorphotoelectrode is low can be deduced as follows. The surface of thesemiconductor film and the powder are not uniform, and many defects,structural steps in atomic level, etc. exist. Therefore, as a result ofthe local surface energy variation depending on the sites on thesurface, differences exist in adsorption performances of carbon dioxide,proton, solvent, gas, and reaction intermediates which are reactants.Therefore, the processes such as probabilities and rates at which theelectron is supplied to these substances are not constant, and variousreaction products are produced.

In addition, the reduction reaction of carbon dioxide requires asemiconductor photocatalyst having a high energy level of the conductionband, and, when a visible-light-responsive semiconductor catalyst havinga narrow bandgap is used, the energy level of the valance band wouldalso necessarily be high, resulting in difficulties in oxidationreaction of water.

Although the reason is not definite, a reason why there are manylimitations in the usage of visible light of a longer wavelength and inphotocatalytic limitations in the complex photocatalyst is that, becausea lifetime of photoexcited electrons is short, a possibility that theelectron moves to the reaction field on the complex is low and the lightabsorption is not efficiently used for the reaction. Because of this, inNon-Patent Document 5, the material cannot function as a photocatalyst.

In Patent Document 6, CZTS, which does not use rare elements and whichis a less expensive electrode material compared to a p-type indiumphosphide (p-InP) or the like, is used, but Patent Document 6 does notdescribe the reduction reaction of carbon dioxide at all. When CZTS isused in an aqueous solution, the hydrogen generation reaction occurspreferentially, and selective reduction of carbon dioxide becomesdifficult.

Solution to Problem

According to one aspect of the present invention, there is provided aphotochemical reaction device comprising an oxidation reaction electrodewhich oxidizes water and generates oxygen, and a reduction reactionelectrode which reduces carbon dioxide and synthesizes a carboncompound, wherein the oxidation reaction electrode and the reductionreaction electrode are electrically connected, and the reductionreaction electrode reduces carbon dioxide and synthesizes the carboncompound in a solution containing water by means of energy of irradiatedlight.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, an energy level of a conduction band ofthe oxidation reaction electrode is positioned at a potential on anegative side in relation to an energy level of a valance band of thereduction reaction electrode.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the reduction reaction electrode has astructure in which a semiconductor electrode and a catalyst whichpresents a reduction action of carbon dioxide are coupled, and thereduction action of carbon dioxide is presented by movement of excitedelectrons generated by radiation of light on the semiconductor electrodeto the catalyst.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the reduction reaction electrode has astructure in which a semiconductor electrode and a catalyst whichpresents a reduction action of carbon dioxide are coupled by chemicalpolymerization, and reduces carbon dioxide and synthesizes the carboncompound in the solution containing water by means of the energy ofirradiated light.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the oxidation reaction electrode and thereduction reaction electrode are placed in a two-chamber cell separatedby a proton exchange membrane, the oxidation reaction electrode and thereduction reaction electrode are electrically connected, and thereduction reaction electrode reduces carbon dioxide and synthesizes thecarbon compound in the solution containing water by means of the energyof irradiated light.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the oxidation reaction electrode and thereduction reaction electrode are electrically connected, the oxidationreaction electrode is a semiconductor electrode, and oxidizes water andtakes away electrons by means of the energy of irradiated light, and thereduction reaction electrode reduces carbon dioxide and synthesizes thecarbon compound in the solution containing water by means of the energyof irradiated light.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the catalyst is a metal complex or apolymer thereof. In particular, the catalyst is preferably a mixture ofa first metal complex having an anchor site which is connected to thesemiconductor electrode and a second metal complex which is polymerizedwith the first metal complex and which has a CO₂ reduction catalyticfunction. Further, the second metal complex preferably has a pyrrolesite.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, a chemical polymerization film of thefirst metal complex and the second metal complex is formed on a surfaceof the semiconductor electrode.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the oxidation reaction electrode and thereduction reaction electrode are directly connected in a state where nobias voltage is applied, and light is irradiated on both electrodes sothat water functions as an electron donor.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the oxidation reaction electrode and thereduction reaction electrode are connected in a state where a bias powersupply is applied, and light is irradiated on both electrodes so thatwater functions as an electron donor.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the oxidation reaction electrodecomprises titanium oxide. In particular, the oxidation reactionelectrode preferably comprises anatase-type titanium oxide.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the solution containing water is water oran aqueous solution containing an electrolyte.

According to another aspect of the present invention, preferably, in thephotochemical reaction device, the oxidation reaction electrode and thereduction reaction electrode are separated by an ion exchange membrane(a cation exchange membrane or an anion exchange membrane).

According to another aspect of the present invention, preferably, in thephotochemical reaction device, there is employed a three-electrodesystem structure which has a reference electrode in addition to theoxidation reaction electrode and the reduction reaction electrode.

According to another aspect of the present invention, there is provideda composite photoelectrode comprising a catalyst which presents areduction action of carbon dioxide and a semiconductor electrode coupledwith the catalyst, wherein the reduction action of carbon dioxide ispresented by movement to the catalyst of excited electrons generated byradiation of light onto the semiconductor electrode.

According to another aspect of the present invention, preferably, in thecomposite photoelectrode, the catalyst is a metal complex or a polymerthereof. Preferably, the semiconductor electrode is a sulfidesemiconductor or a phosphide semiconductor.

According to another aspect of the present invention, there is provideda light energy storage device in which the composite photoelectrode andan oxidation reaction electrode which oxidizes water and generatesoxygen are connected.

Advantageous Effects of Invention

According to various aspects of the present invention, carbon dioxidecan be reduced and a useful carbon compound can be synthesized by meansof light energy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing a structure of a photochemicalreaction device according to an embodiment of an electric bias of 0.

FIG. 2 is a diagram schematically showing a structure of a photochemicalreaction device according to an embodiment in which an electric bias isapplied.

FIG. 3 is a diagram showing a structural formula of a Ru complex and astructural formula of a polymerized Ru complex.

FIG. 4 is a diagram showing a current-voltage characteristic of adevice.

FIG. 5 is a partial enlarged view of FIG. 4.

FIG. 6 is a diagram showing an example metal complex according to apreferred embodiment of the present invention.

FIG. 7 is a diagram showing a structure of a photochemical reactiondevice of a three-electrode system in an example of the presentinvention.

FIG. 8 is a diagram showing an example metal complex according to apreferred embodiment of the present invention.

FIG. 9 is a diagram showing a result of time-of-flight secondary ionmass spectrometry showing adsorption of a ligand to a semiconductorsubstrate.

FIG. 10 is a diagram showing a result of time-of-flight secondary ionmass spectroscopy showing adsorption of a ligand to a semiconductorsubstrate.

FIG. 11 is a diagram showing a structure of a photoelectrochemicalmeasurement device in an example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention will now be describedwith reference to the drawings.

FIG. 1 shows a structure of a photochemical reaction device according toa preferred embodiment of the present invention. A reduction reactionelectrode 10 which is a semiconductor electrode and an oxidationreaction electrode 12 which is a counter electrode of the reductionreaction electrode and which is a semiconductor electrode areelectrically connected. As shown in FIG. 2, it is preferable to providea bias power supply 14 between the reduction reaction electrode 10 andthe oxidation reaction electrode 12 so that a negative electric bias ofa voltage (0-1.4 V) is applied to the reduction reaction electrode 10with respect to the oxidation reaction electrode 12.

A catalyst of a base material 16 contacts the reduction reactionelectrode 10 in a state where electrons e⁻ can be exchanged. In theexample structure shown in the figures, a metal complex (rutheniumcomplex) is used as the base material 16.

In such a system, when light is irradiated onto the reduction reactionelectrode 10, photoexcited electrons e⁻ are generated, and are used forthe reduction catalytic reaction of the base material 16. In theillustrated example, carbon dioxide (CO₂) is reduced to formic acid(HCOOH).

Similarly, when light is irradiated onto the oxidation reactionelectrode 12, reactions occur in which water (H₂O) is oxidized to oxygen((1/2)O₂) or hydrogen peroxide by photocatalytic reactions, and thegenerated electrons e⁻ move to the reduction reaction electrode 10 andare combined with holes that are generated as a pair for thephotoexcited electrons in the reduction reaction electrode 10.

As described, in the present embodiment, the photoexcited electrons e⁻generated in the reduction reaction electrode 10 by the radiation oflight move to the reaction sites of the base material 16, which presentsthe reduction action of carbon dioxide so that the reduction reaction ofcarbon dioxide takes place. In particular, because the photoexcitedelectrons are used, carbon dioxide can be reduced and useful organiccompounds can be synthesized with high efficiency and with a highreaction product selectivity, without application of a bias voltage. Inaddition, oxidation of water can be realized using holes generated byradiation of light onto the oxidation reaction electrode 12. Theelectrons generated here efficiently combine with the holes generated atthe reduction reaction electrode 10. Because of this, even in theabsence of the bias power supply 14, the reduction reaction of carbondioxide can proceed with the water used as the electron donor. Thereaction can be more efficiently promoted by placing the bias powersupply 14 and applying a bias voltage between the electrodes.

By employing a two-electrode system in which the reduction reactionelectrode 10 and the oxidation reaction electrode 12 are providedseparately, it is possible to reduce the carbon dioxide using water asthe electron donor, and the energy necessary for the oxidation reactionof water and the reduction reaction of carbon dioxide can be dividedinto two. Therefore, use of a visible-light-responsive semiconductormaterial which can absorb only light of a narrow light wavelength regionis enabled.

[Reduction Reaction Electrode]

Here, as a semiconductor used for the reduction reaction electrode 10,there is used a material in which a value obtained by subtracting avalue of a level having the lowest energy among molecular orbitals, ofthe base material to be described later, not occupied by the electronsfrom a value of the lowermost energy level of the conduction band isless than or equal to 0.2 electron volts. For example, the semiconductormay be tantalum oxide, a nitride semiconductor such as tantalum nitride,nitrogen-doped tantalum oxide, etc., tantalum oxynitride, a sulfidesemiconductor such as nickel-containing zinc sulfide, copper-containingzinc sulfide, zinc nitride, etc., a selenide semiconductor such ascadmium selenide, a chalcogenide semiconductor including telluriumcompounds and other composite compounds, a phosphide semiconductor(phosphorous compound) such as indium phosphide, gallium phosphide,indium gallium phosphide, etc., iron oxide, silicon carbide, an oxide ofcopper, an arsenide semiconductor such as gallium arsenide,rhodium-doped strontium titanate, or the like.

Alternatively, the semiconductor used for the reduction reactionelectrode 10 may be a sulfide semiconductor containing copper, zinc,tin, and sulfur such as Cu₂ZnSnS₄ (CZTS) or Cu₂ZnSn(S,Se)₄ (CZTSSe).When a compound semiconductor suitable for the semiconductor to be usedfor the reduction reaction electrode 10 is represented byA_(x)B_(y)C_(z)D₄, copper or silver is preferably used as A, zinc orcadmium is preferably used as B, tin, germanium, gallium, or aluminum ispreferably used as C, and sulfur, oxygen, selenium, or the like ispreferably used as D. The composition conditions are 1.4≦x/y≦2 and1.4≦x/z≦2, and preferably, 70% or more of A is copper, 90% or more ofBis zinc, 90% of C is tin and germanium, and 70% or more of D is sulfurand selenium.

Here, zinc-doped indium phosphide and sulfide semiconductors containingcopper, zinc, tin, and sulfur (for example, CZTS, CZTSSe, or the like)which are used in the Examples of the present invention to be describedlater are particularly preferable for the reduction reaction electrode10. For the zinc-doped indium phosphide, there is used, for example, astructure synthesized by the Vapor controlled Czochralski (VCZ) method,the LEC method, or the HB method.

Tantalum nitride and tantalum oxynitride can be generated by thermallyprocessing tantalum oxide in an atmosphere containing ammonia gas. Theammonia is preferably diluted with non-oxidizing gas (such as argon andnitrogen), and, for example, it is preferable to place and heat tantalumoxide in a gas flow in which ammonia and argon are mixed at equal flowrates. The heating temperature is preferably greater than or equal to500° C. and less than or equal to 900° C., more preferably, greater thanor equal to 550° C. and less than or equal to 850° C. The processingperiod is preferably greater than or equal to 1 hour and less than orequal to 15 hours. For the tantalum oxide before the ammonia processing,there may be used commercially available tantalum oxide havingcrystallinity or amorphous tantalum oxide obtained by applying ahydrolysis process or the like to a compound solution containingtantalum such as tantalum chloride.

The nickel-containing zinc sulfide can be obtained by dissolving anickel-containing hydrate and a zinc-containing hydrate, introducingaqueous solution to which a sodium sulfide hydrate is dissolved,stirring, applying centrifugal separation and re-dispersion, removingthe supernatant liquor, and drying. The nickel-containing hydrate maybe, for example, nickel (II) nitrate hexahydrate. The zinc-containinghydrate may be, for example, zinc (II) nitrate hexahydrate. Here, as anickel source, in addition to the above-described source, nickelchloride, nickel acetate, nickel perchlorate, nickel sulfate, or thelike may be used. Similarly, as a zinc source, zinc chloride, zincacetate, zinc perchlorate, zinc sulfate, or the like may be used.

Similarly, copper-containing zinc sulfide can be obtained by dissolvinga copper-containing hydrate and a zinc nitrate-containing hydrate,introducing a sodium sulfide hydrate, stirring, applying centrifugalseparation and re-dispersion, removing the supernatant liquor, anddrying. The copper-containing hydrate may be, for example, copper (II)nitrate 2.5-hydrate. The zinc-containing hydrate may be, for example,zinc (II) nitrate hexahydrate. Here, as a copper source, in addition tothe above-described source, copper chloride, copper acetate, copperperchlorate, copper sulfate, or the like may be used. Similarly, as azinc source, zinc chloride, zinc acetate, zinc perchlorate, zincsulfate, or the like may be used.

[Base Material]

As the base material 16, there is used a material in which a valueobtained by subtracting a value of a level of the lowest energy amongthe molecular orbitals, of the base material 16, not occupied by theelectrons from a value of the lowermost energy level of the conductionband of the semiconductor of the reduction reaction electrode 10 is lessthan or equal to 0.2 electron volts. The base material 16 may be a metalcomplex or a polymer thereof, and, for example, there are used a rheniumcomplex having a carboxybipyridine ligand ((Re(dcbpy)(CO)₃P(OEt)₃)),((Re(dcbpy)(CO)₃Cl)), Re(dcbpy)(CO)₃MeCN, or Re(dcbqi)(CO)₃MeCN, and aruthenium (Ru) complex [Ru(dcbpy)(bpy)(CO)₂]²⁺ (bpy=2,2′-bipyridine,dcbpy=4,4′-dicarboxy-2,2′-bipyridine).

In particular, a ruthenium (Ru) complex or a polymer thereof used in theExample of the present invention is preferable. In particular,[Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)₂]_(n) asshown in FIG. 3( b) obtained by polymerizing[Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)₂Cl₂]shown in FIG. 3( a) is preferably used for formation of the reductionreaction electrode 10. In addition, the use of[Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(CH₃CN)Cl₂] is also preferable.

In the base material 16, no particular limitation is imposed on themethod of synthesizing the complex polymer, so long as the polymercontains a metal complex which shows a reduction activity of carbondioxide. For example, the methods may be (1) chemical polymerization inwhich the polymerization is realized through a chemical reaction, (2)electrolytic polymerization in which the polymerization is realizedthrough an electrochemical reaction, or (3) photochemical polymerizationand photoelectrochemical polymerization which use light for theabove-described reactions. No particular limitation is imposed on themethod of modifying the complex polymer on the reduction reactionelectrode, and, for example, (1) spin coating, (2) dip coating, (3)spraying, (4) dropping, or the like may be employed.

The reduction reaction electrode 10 and the base material 16 arecoexistent such that electrons can be exchanged. For example, the basematerial 16 may float in an electrolytic solution or may be coupled.When the base material 16 is coupled, for example, the base material 16is mixed in the solvent. The solvent is dropped on the reductionreaction electrode 10, to adhere the base material 16 on the surface ofthe electrode. The photoelectrode can be obtained by a method, forexample, in which the base material 16 is coupled on the surface of thereduction reaction electrode 10 by drying the adhered material. For thesolvent, an organic solvent may be employed, and, for example,acetonitorile, methanol, ethanol, acetone, or the like may be employed.

The base material 16 may be any compound which shows a carbon dioxidereduction activity using electrons, and no particular limitation isimposed thereon. In the case of a metal complex, a metal complex of atleast one metal selected from the VII family metals and the VIII familymetals on the periodic table may be used. For example, there may be useda complex of a metal such as ruthenium, rhenium, manganese, iron,copper, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum,or the like and a ligand. Examples of the metal complex include[Re(N—N)(CO)₃L] (N—N=a complex compound containing nitrogen)(L=unidentate ligand such as PR₃, SCN, Cl, MeCN, and DMF), [Ru(N—N)(N′—N′)(CO)₂], [Ru(tpy)(N—N)(CO)], [Cu(N—N)₂], [Co(N—N)₃], [Fe(N—N)₃],[Ni(N—N)₃], etc.

No particular limitation is imposed on the ligand, and examples oftypical primary ligands include a heterocyclic compound containingnitrogen, a heterocyclic compound containing oxygen, and a heterocycliccompound containing sulfur. Preferable examples of secondary ligandsinclude CO, halogens, phosphines, or the like. In addition, solventmolecules such as acetonitrile, DMF, water, or the like may also beused. These secondary ligands may be converted and generated during thereaction. One or a combination of two or more of these ligands may beused.

As the heterocyclic compound containing nitrogen, for example, pyridine,bipyridine, phenanthroline, terpyridine, pyrrole, indole, carbazole,imidazole, pyrazole, quinoline, isoquinoline, acridine, pyridazine,pyrimidine, pyrazine, phthalazine, quinazoline, quinoxaline, or the likecan be exemplified. As the heterocyclic compound containing oxygen, forexample, furan, benzofuran, oxazole, pyran, pyrone, coumarin,benzopyrone, or the like can be exemplified. As the heterocycliccompound containing sulfur, for example, thiophene, thionaphthene,thiazole, or the like can be exemplified. Such a ligand may be used as asingle entity or in a combination of two or more ligands.

The reduction reaction electrode 10 and the base material 16 arepreferably chemically bonded by a linking group. No particularlimitation is imposed on the linking group, and it may be any groupwhich enables chemical bonding. Examples include a carboxyl group, aphosphate group, a sulfonic acid group, a silanol group, a thiol group,and derivatives thereof. Here, in the state where the linking group islinked to the reduction reaction electrode 10, the linking group mayhave a structure in which a proton is detached or a structure in which ametal and an oxygen atom are coordinate bonded. These linking groups maybe used as a single entity or in a combination of two or more linkinggroups. Alternatively, a plurality of linking groups may be used.

The linking method of the reduction reaction electrode 10 and the basematerial 16 may be any method which chemically bonds the semiconductorof the reduction reaction electrode 10 and the base material, and noparticular limitation is imposed thereon. For example, the method may be(1) adhering a metal complex in which a linking group is introduced intothe ligand which is adsorbed on the semiconductor, (2) directly formingthe complex after the ligand to which the linking group is introduced isadsorbed on the semiconductor, or (3) bonding the metal complex to thesemiconductor to which the linking group is introduced.

A coverage of the base material 16 is preferably greater than or equalto 1% and less than or equal to 100% with respect to a surface area ofthe reduction reaction electrode 10. When the coverage of the basematerial 16 is less than 1%, the amount of the base material is too low,and sufficient carbon dioxide reduction activity cannot be expressed.

A particularly preferable combination is, in a case of the semiconductorbeing a metal oxide, a phosphate group serving as the linking group,and, in a case of the semiconductor being a compound semiconductor suchas GaP and InP, ester phosphate serving as the linking group.

As will be described below in the Examples of the present invention, itis preferable to electrochemically deposit a Ru complex on the reductionreaction electrode 10. The reduction reaction electrode 10 and thecounter electrode can be immersed, and the Ru complex can be bonded onthe reduction reaction electrode 10 by electrodeposition.

Alternatively, the use of a metal complex having an anchor ligand isalso preferable. Examples of the metal complex having an anchor ligandinclude [Ru({4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂)] and[Ru({4,4′-dicarboxylic acid-2,2′-bipyridine}(CO)₂Cl₂)], or the like. Inaddition to these, a metal complex including a pyrrole ligand may bepolymerized and linked to the reduction reaction electrode 10, toimprove the catalytic performance of the reduction reaction of carbondioxide. In addition, selectivity of the reduction reaction can also beimproved.

[Oxidation Reaction Electrode]

For the oxidation reaction electrode 12, there is used a material whichexhibits a photocatalytic function and causes the oxidation reaction ofwater with radiation of light. For example, titanium oxide (TiO₂),nitrogen-doped titanium oxide (N—TiO₂), rutile-type titanium oxide,tungsten oxide (WO₃), strontium titanate, tantalum oxynitride (TaON), abismuth vanadate compound, or the like is used. These materials areformed by direct synthesis such as sputtering, hydrolysis, andpolymerization, or by a method of fixing powders with a binder, or thelike. These materials are used as a single entity or in a configurationof being formed on a conductive substrate. Commercially availabletitanium oxide particles (TiO₂(P25)) and TiO₂, obtained by reducingtitanium oxide by hydrogen, which are used in the Examples of thepresent invention, are particularly preferable.

In such a photochemical reaction device, for example, the reductionreaction electrode 10 and the oxidation reaction electrode 12 areimmersed in water in which carbon dioxide is dissolved, and light isirradiated on both electrodes 10 and 12. With this process, as describedabove, the reduction catalytic reaction at the base material 16 causesproduction of formic acid from carbon dioxide in the water, and water isoxidized to oxygen gas at the oxidation reaction electrode 12 by meansof a photocatalytic reaction. The product is not limited to formic acid,and, by selecting the base material 16 and causing the catalyticreaction at a suitable environment, it is also possible to synthesizeuseful organic substances such as alcohol from carbon dioxide.

When the bias power supply 14 is provided and the bias voltage isapplied as shown in FIG. 2, the operation can employ a lower biasvoltage (0-1.4 V) than in the case of a two-electrode system usingnon-photoelectrode material. This is because photoexcitation by lightenergy is used in the reduction reaction electrode 10 and the oxidationreaction electrode 12.

As described, according to the present embodiment, by means of lightenergy, carbon dioxide can be converted into useful carbon compounds andthe light energy can be stored in the carbon compound. In particular,because carbon dioxide can be reduced using water as the electron donor,there is obtained an advantage in that the cost of the overall systemcan be reduced. In addition, with the use of a complex catalyst whichpresents a reduction action of carbon dioxide, the carbon compound canbe synthesized with a high reaction product selectivity.

By using the light energy, carbon dioxide can be reduced with a lowervoltage than can the non-photoelectrode material. By using thetwo-electrode system of the reduction reaction electrode 10 and theoxidation reaction electrode 12, the reaction fields of reduction andoxidation can be separated, and products can be easily separated. Inaddition, by using photoelectrodes for both electrodes, light energynecessary for the oxidation reaction of water and the reduction reactionof carbon dioxide can be absorbed and used over a wide wavelength range.

Example 1

With a device shown in FIG. 2 (including a bias voltage of 0 V),experiments were performed. First, for photoelectrochemical measurement,an electrochemical analyzer (BAS) was used and measurement was conductedwith the two-electrode system. A Pyrex (registered trademark) glass cellwas used for the container. A xenon lamp (MAX-302 manufactured by AsahiSpectra) was used as the light source.

For evaluation of the products involved with the photoelectrochemicalmeasurement, ion chromatograph (DIONEX with ICS-2000 auto-sampler AS)was used. For the column of the ion chromatograph, “IonPac AS15” wasused, a KOH eluate was used for the eluate, and an electric conductivitydetector was used for the detector.

Example 1

In an acetonitrile solution containing about 1 mg of a ruthenium complex[Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)₂Cl₂](FIG. 3( a)), zinc-doped indium phosphide (p-InP) synthesized through aVCZ method (having a carrier concentration of 4×10¹⁸-6×10¹⁸/cm³) wasused for a working electrode, platinum was used for a counter electrode,an I⁻/I³⁻ electrode was used for a reference electrode, and argon gaswas passed for 10 minutes. A potential of −1.2 V with respect to thereference electrode was applied, and electrodeposition was performed forone hour under irradiation with a fluorescent lamp, to deposit aruthenium complex polymer [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)₂]_(n) (FIG. 3( b)) on the surface of theworking electrode.

A carbon dioxide reduction reaction was performed using theabove-described zinc-doped indium phosphide on which the rutheniumcomplex polymer was deposited (p-InP—Zn (Ru-polymer)) as the workingelectrode (reduction reaction electrode 10). For the counter electrode(oxygen reaction electrode 12), there was used a rutile monocrystallinetitanium oxide electrode (TiO_(2-x)), to which a reduction process wasapplied with hydrogen. As the electrolyte, 8 ml of distilled water wasused. After argon gas was bubbled in the solution for about 20 minutesto remove the dissolved gas, carbon dioxide gas was bubbled in thesolution for about 10 minutes, and then, a current-voltage measurementwas conducted under a carbon dioxide gas atmosphere. Finally, under acarbon dioxide gas atmosphere, a current-time measurement was conductedwhile a bias voltage of −0.8 V was applied with respect to the reductionreaction electrode 10 and the oxidation reaction electrode 12.

Example 2

In the configuration of Example 1, the current-time measurement wasconducted while a bias voltage of −0.4 V was applied with respect to thereduction reaction electrode 10 and the oxidation reaction electrode 12.

Example 3

In the configuration of Example 1, a tungsten oxide electrode (WO₃) wasused for the oxidation reaction electrode 12, and a cut-off filter ofλ>422 nm was used for the light source so that only visible light wasirradiated, and the current-time measurement was conducted while a biasvoltage of −0.8 V was applied with respect to the reduction reactionelectrode 10 and the oxidation reaction electrode 12.

Comparative Example 1

In the configuration of Example 1, carbon dioxide gas was not bubbledand the current-time measurement was conducted under an argon gasatmosphere.

Comparative Example 2

In the configuration of Example 2, carbon dioxide gas was not bubbledand the current-time measurement was conducted under the argon gasatmosphere.

Comparative Example 3

In the configuration of Example 1, there was used a wafer (8 mm×20 mm)of zinc-doped indium phosphide (p-InP—Zn) on which the Ru complexpolymer was not electrodeposited, and the current-time measurement wasconducted under the carbon dioxide gas atmosphere.

[Result]

TABLE 1 collectively shows conditions and results of Examples 1-3 andComparative Examples 1-3.

TABLE 1 WORKING COUNTER LIGHT ELECTRODE ELECTRODE ELECTROLYTE GAS SOURCEEXAMPLE 1 p-InP TiO₂ H₂O CO₂ ALL LIGHTS Ru polymer EXAMPLE 2 p-InP TiO₂H₂O CO₂ ALL LIGHTS Ru polymer EXAMPLE 3 p-InP WO₃ H₂O CO₂ VISIBLE LIGHTRu polymer (λ > 422 nm) COMPARATIVE p-InP TiO₂ H₂O Ar ALL LIGHTS EXAMPLE1 Ru polymer COMPARATIVE p-InP TiO₂ H₂O Ar ALL LIGHTS EXAMPLE 2 Rupolymer COMPARATIVE p-InP TiO₂ H₂O CO₂ ALL LIGHTS EXAMPLE 3 POTENTIALFORMIC ACID FARADAIC DIFFERENCE TIME CONCENTRATION CHARGE EFFICIENCY (Vvs CE) (hour) (μM) (C) (%) EXAMPLE 1 −0.8 20 31 1.26 3.8 EXAMPLE 2 −0.420 4 0.85 0.7 EXAMPLE 3 −0.8 20 19 1.18 2.5 COMPARATIVE −0.8 20 5 0.243.3 EXAMPLE 1 COMPARATIVE −0.4 20 1 0.16 0.9 EXAMPLE 2 COMPARATIVE −0.820 0 0.73 0.1 EXAMPLE 3

While 31 μM of formic acid was detected in 20 hours under the carbondioxide gas atmosphere as a result of the current-time measurement withapplication of the bias voltage of −0.8 V in Example 1, only 5 μM of theformic acid was detected in 20 hours under the argon gas atmosphere ofComparative Example 1.

While 4 μM of formic aced was detected in 20 hours under the carbondioxide atmosphere in Example 2 in which the time-current measurementwas conducted with application of the bias voltage of −0.4 V, only 1 μMof formic acid was detected in 20 hours under the argon gas atmospherein Comparative Example 2.

In addition, only 0.3 μM of formic acid was detected in 20 hours also inthe case of Comparative Example 3 where only the indium phosphideelectrode was used under the carbon dioxide gas atmosphere. Based onthis, it was suggested that carbon dioxide is reduced to formic acid onthe indium phosphide electrode on which the Ru complex polymer isadhered in the distilled water which does not use the electrolytes.

In Example 3 in which the counter electrode was changed from the TiO₂,photoelectrode to the WO₃ photoelectrode and the current-timemeasurement was conducted under irradiation with visible light of λ>422nm and the bias voltage of −0.8 V, 19 μM of formic acid was detected in20 hours under the carbon dioxide gas atmosphere. Thus, it was suggestedthat carbon dioxide was reduced to formic acid even when only thevisible light was irradiated.

FIGS. 4 and 5 show results of measurements of current for cases wherebias voltage was changed in the configurations of Example 1 andComparative Example 1, and in configurations where no light wasirradiated in the configurations of Example 1 and Comparative Example 1.When no light was irradiated, no current flowed. On the other hand, whenlight was irradiated, a photocurrent flows, but, in a condition with thepresence of carbon dioxide as in Example 1, the photocurrent is largerthan that in the case of the argon gas atmosphere. In particular, thephotocurrent is increased even in the case of the bias voltage of 0 V,due to the presence of carbon dioxide. Based on this, it was found thatthe reduction reaction of carbon dioxide took place even with the biasvoltage of 0 V. Thus, it was suggested that the p-InP—Zn(Ru-polymer)/TiO_(2-x) based devices can be operated with a zero bias.

In addition, as a result of the current-voltage measurement under thecarbon dioxide gas atmosphere in Example 1, it was confirmed that avalue of the photocurrent was increased as the absolute value of thenegative bias voltage applied to the reduction reaction electrode 10with respect to the oxidation reaction electrode 12 was increased.

Example 4

0.25 mg of a ruthenium complex [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine](CO)(CH₃CN)Cl₂] was dissolved in 0.25 ml ofan acetonitrile solution, 5 μl of pyrrole solution (having a molar ratioof pyrrole with respect to the ruthenium complex of 1.1%) was mixed, andthen, 5 μl of 0.2 M iron (III) chloride solution (having a molar ratioof iron chloride with respect to the ruthenium complex of a factor of3.1) was added. The pyrrole solution was prepared by diluting 50 μl ofpyrrole by 1 ml of acetonitrile. The iron (III) chloride solution wasprepared by dissolving 1.08 g of iron (III) chloride hexahydrate into 20ml of ethanol. 50 μl of the above-described mixture solution was appliedon a p-InP—Zn photoelectrode, and dried in an oven at 45° C. Suchapplication and drying were repeated for 5 times, to create a Ru-polymer(CP)/p-InP—Zn photoelectrode. The Ru-polymer (CP)/p-InP—Znphotoelectrode thus created was used as the working electrode (reductionreaction electrode 10).

In addition, 30 μl of acetyl acetone, 400 μl of water, and one drop ofsurfactant (Triton X-100) were well-kneaded with 0.2 g of particles ofcommercially available titanium oxide particle (TiO₂(P25)), to prepare apaste, and the paste was dropped on a glass substrate on which atransparent conductive layer was provided, applied and rolled with aglass rod, dried, and then sintered at a temperature of 550° C., tocreate a TiO₂(P25) photoelectrode. The TiO₂(P25) photoelectrode thuscreated was used as the counter electrode (oxidation reaction electrode12).

The above-described Ru-polymer (CP)/p-InP—Zn photoelectrode (reductionreaction electrode 10) and the TiO₂(P25) photoelectrode (oxidationreaction electrode 12) were placed in chambers of a two-chamber cellseparated by a proton exchange membrane (Nafion 117). Pure water wasused for the electrolyte. Such a photochemical reaction device wasplaced under a carbon dioxide gas atmosphere, and the current-timemeasurement was conducted while radiating light in a state where no biasvoltage was applied to the reduction reaction electrode 10 and theoxidation reaction electrode 12.

[Result]

In Example 4, 0.26 C of charges were observed and 115 μM of formic acidwas detected when light corresponding to 1.4 SUN was irradiated for 20hours. A ratio of the produced formic acid with respect to the amount ofobserved charges (Faradaic efficiency) was calculated and found to be35.8%. Even in comparison to Example 1, characteristics of both theamount of production of formic acid and the Faradaic efficiency weresignificantly improved even though no bias voltage was applied.

The following three reasons can be considered as reasons why thecharacteristics were improved: (1) because the reaction fields wereseparated using the two-chamber cell, formic acid produced on the sideof the reduction reaction could be efficiently accumulated and recoveredwithout the formic acid being decomposed on the side of the oxidationreaction; (2) because the TiO₂, photoelectrode was changed to theTiO₂(P25) photoelectrode, the current value at the zero-bias conditionwas improved by a factor of approximately 4; and (3) because the methodof modifying the ruthenium complex polymer was changed, the amount ofproduction of formic acid was improved by a factor of approximately 4.

Example 5

A MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6(a)).FeCl₃.pyrrol was applied to a wafer (8 mm×20 mm) of zinc-dopedindium phosphide (p-InP—Zn manufactured by Sumitomo Electric Industries)which is a p-type semiconductor, dried, and then washed with water. Thiselectrode was used as the working electrode. In the present example, athree-electrode system as shown in FIG. 7 was employed, a glassy carbonelectrode (GC) was used for the counter electrode, and a silver/silverchloride electrode (Ag/AgCl) was used for the reference electrode. Forthe electrolyte, 5 ml of distilled water was used. After argon gas wasbubbled in the solution for about 20 minutes to remove the dissolvedgas, carbon dioxide gas was bubbled in the solution for about 10minutes, and then, light was irradiated under a carbon dioxide gasatmosphere, and reduction and oxidation reactions were measured. For thepotential, −0.4 V was applied with respect to the reference electrode.

Example 6

An MeCN solution including FeCl₃.pyrrol in which[Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and [Ru{4,4′-diphosphateethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6( b)) were mixed in a1:1 ratio was applied on a wafer (8 mm×20 mm) of zinc-doped indiumphosphide (p-InP—Zn manufactured by Sumitomo Electric Industries), whichis a p-type semiconductor, dried, and then washed with water. Thiselectrode was used as the working electrode. In the present example, athree-electrode system as shown in FIG. 7 was employed, the glassycarbon electrode was used for the counter electrode, and thesilver/silver chloride electrode (Ag/AgCl) was used for the referenceelectrode. For the electrolyte, 5 ml of distilled water was used. Afterargon gas was bubbled in the solution for about 20 minutes to remove thedissolved gas, carbon dioxide gas was bubbled in the solution for about10 minutes, and then, light was irradiated under a carbon dioxide gasatmosphere and reduction and oxidation reactions were measured. For thepotential, −0.4 V was applied with respect to the reference electrode.

Example 7

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO) (MeCN)Cl₂] (refer to FIG. 6(a)).FeCl₃.pyrrol was applied on a wafer (8 mm×20 mm) of zinc-dopedgallium phosphide (p-GaP—Zn manufactured by Sumitomo ElectricIndustries), which is a p-type semiconductor, dried, and then washedwith water. This electrode was used as the working electrode. In thepresent example, a three-electrode system as shown in FIG. 7 wasemployed, the glassy carbon electrode was used for the counterelectrode, and the silver/silver chloride electrode (Ag/AgCl) was usedfor the reference electrode. For the electrolyte, 5 ml of distilledwater was used. After argon gas was bubbled in the solution for about 20minutes to remove the dissolved gas, carbon dioxide gas was bubbled forabout 10 minutes, and then, light was irradiated under the carbondioxide gas atmosphere and the reduction and oxidation reactions weremeasured. For the potential, −0.4 V was applied with respect to thereference electrode.

Example 8

An MeCN solution containing FeCl₃.pyrrol in which[Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and [Ru{4,4′-diphosphateethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a 1:1ratio was applied on a wafer (8 mm×20 mm) of zinc-doped galliumphosphide (p-GaP—Zn manufactured by Sumitomo Electric Industries), whichis a p-type semiconductor, dried, and then washed with water. Thiselectrode was used as the working electrode. In the present embodiment,the three-electrode system as shown in FIG. 7 was employed, the glassycarbon electrode was used for the counter electrode, and thesilver/silver chloride electrode (Ag/AgCl) was used for the referenceelectrode. For the electrolyte, 5 ml of distilled water was used. Afterargon gas was bubbled in the solution for about 20 minutes to remove thedissolved gas, carbon dioxide gas was bubbled in the solution for about10 minutes, and then, light was irradiated under the carbon dioxide gasatmosphere and the reduction and oxidation reactions were measured. Forthe potential, −0.4 V was applied with respect to the referenceelectrode.

Example 9

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6(a)).FeCl₃.pyrrol was applied on a wafer (8 mm×20 mm) of silicon (p-Si),which is a p-type semiconductor, dried, and then washed with water. Thiselectrode was used as the working electrode. In the present example, thethree-electrode system as shown in FIG. 7 was employed, the glassycarbon electrode was used for the counter electrode, and thesilver/silver chloride electrode (Ag/AgCl) was used for the referenceelectrode. For the electrolyte, 5 ml of distilled water was used. Afterargon gas was bubbled in the solution for about 20 minutes to remove thedissolved gas, carbon dioxide gas was bubbled in the solution for about10 minutes, and then, light was irradiated under the carbon dioxide gasatmosphere, and the reduction and oxidation reactions were measured. Forthe potential, −0.4 V was applied with respect to the referenceelectrode.

Example 10

An MeCN solution containing FeCl₃.pyrrol in which[Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and[Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a wafer (8 mm×20 mm) ofsilicon (p-Si), which is a p-type semiconductor, dried, and then, washedwith water. This electrode was used as the working electrode. In thepresent example, the three-electrode system as shown in FIG. 7 wasemployed, the glassy carbon electrode was used for the counterelectrode, and the silver/silver chloride electrode (Ag/AgCl) was usedfor the reference electrode. For the electrolyte, 5 ml of distilledwater was used. After argon gas was bubbled in the solution for about 20minutes to remove the dissolved gas, carbon dioxide gas was bubbled inthe solution for about 10 minutes, and then, light was irradiated underthe carbon dioxide gas atmosphere and reduction and oxidation reactionswere measured. For the potential, −0.4 V was applied with respect to thereference electrode.

Example 11

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6(a)).FeCl₃.pyrrol was applied on a sputtered film (20 mm×20 mm) ofnitrogen-doped tantalum oxide (N—Ta₂O₅), which is a p-typesemiconductor, dried, and then washed with water. This electrode wasused as the working electrode. In the present example, thethree-electrode system as shown in FIG. 7 was employed, the glassycarbon electrode was used for the counter electrode, and thesilver/silver chloride electrode (Ag/AgCl) was used for the referenceelectrode. For the electrolyte, 5 ml of distilled water was used. Afterargon gas was bubbled for about 20 minutes to remove the dissolved gas,carbon dioxide gas was bubbled in the solution for about 10 minutes, andthen, light was irradiated under the carbon dioxide gas atmosphere andthe reduction and oxidation reactions were measured. For the potential,−0.4 V was applied with respect to the reference electrode.

Example 12

An MeCN solution containing FeCl₃.pyrrol in which[Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and[Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a sputtered film (20 mm×20mm) of nitrogen-doped tantalum oxide (N—Ta₂O₅), which is a p-typesemiconductor, dried, and then washed with water. This electrode wasused as the working electrode. In the present example, thethree-electrode system as shown in FIG. 7 was employed, the glassycarbon electrode was used for the counter electrode, and thesilver/silver chloride electrode (Ag/AgCl) was used for the referenceelectrode. For the electrolyte, 5 ml of distilled water was used. Afterargon gas was bubbled in the solution for about 20 minutes to remove thedissolved gas, carbon dioxide gas was bubbled in the solution for about10 minutes, and then, light was irradiated under the carbon dioxide gasatmosphere and the reduction and oxidation reactions were measured. Forthe potential, −0.4 V was applied with respect to the referenceelectrode.

Example 13

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6(a)).FeCl₃.pyrrol was applied on a wafer (8 mm×20 mm) of zinc-dopedindium phosphide (p-InP—Zn manufactured by Sumitomo ElectricIndustries), which is a p-type semiconductor, dried, and then washedwith water. This electrode was used as the working electrode. In thepresent example, the three-electrode system as shown in FIG. 7 wasemployed, the glassy carbon electrode was used for the counterelectrode, and the silver/silver chloride electrode (Ag/AgCl) was usedfor the reference electrode. For the electrolyte, 5 ml of 10 mM aqueoussolution of NaHCO₃ was used. After argon gas was bubbled in the solutionfor about 20 minutes to remove the dissolved gas, carbon dioxide gas wasbubbled in the solution for about 10 minutes, and then, light wasirradiated under the carbon dioxide gas atmosphere, and the reductionand oxidation reactions were measured. For the potential, −0.4 V wasapplied with respect to the reference electrode.

Example 14

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6(a)).FeCl₃.pyrrol was applied on a wafer (8 mm×20 mm) of zinc-dopedindium phosphide (p-InP—Zn manufactured by Sumitomo ElectricIndustries), which is a p-type semiconductor, dried, and then washedwith water. This electrode was used as the working electrode. In thepresent example, the three-electrode system as shown in FIG. 7 wasemployed, the glassy carbon electrode was used for the counterelectrode, and the silver/silver chloride electrode (Ag/AgCl) was usedfor the reference electrode. For the electrolyte, 5 ml of 10 mM aqueoussolution of Na₃PO₄ was used. After argon gas was bubbled in the solutionfor about 20 minutes to remove the dissolved gas, carbon dioxide gas wasbubbled in the solution for about 10 minutes, and then, light wasirradiated under the carbon dioxide gas atmosphere and the reduction andoxidation reactions were measured. For the potential, −0.4 V was appliedwith respect to the reference electrode.

Example 15

An MeCN solution containing [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6(a)).FeCl₃.pyrrol was applied on a wafer (8 mm×20 mm) of zinc-dopedindium phosphide (p-InP—Zn manufactured by Sumitomo ElectricIndustries), which is a p-type semiconductor, dried, and then washedwith water. This electrode was used as the working electrode. In thepresent example, the three-electrode system as shown in FIG. 7 wasemployed, the glassy carbon electrode was used for the counterelectrode, and the silver/silver chloride electrode (Ag/AgCl) was usedfor the reference electrode. For the electrolyte, 5 ml of a 10 mMaqueous solution of Na₂SO₄ was used. After argon gas was bubbled in thesolution for about 20 minutes to remove the dissolved gas, carbondioxide gas was bubbled in the solution for about 10 minutes, and then,light was irradiated under the carbon dioxide gas atmosphere and thereduction and oxidation reactions were measured. For the potential, −0.4V was applied with respect to the reference electrode.

Example 16

In the configuration of Example 6, [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and[Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a ratio of 1:4 and applied, and a catalytic activitywas measured.

Example 17

In the configuration of Example 6, [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN) Cl₂] (refer to FIG. 6( a)) and[Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a ratio of 4:1 and applied, and a catalytic activitywas measured.

Example 18

In the configuration of Example 6, [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and[Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a ratio of 9:1 and applied, and a catalytic activitywas measured.

Comparative Example 4

In the configuration of Example 5, the complex catalyst was not appliedand the catalytic activity was measured with only the semiconductor.

Comparative Example 5

In the configuration of Example 7, the complex catalyst was not appliedand the catalytic activity was measured with only the semiconductor.

Comparative Example 6

In the configuration of Example 9, the complex catalyst was not appliedand the catalytic activity was measured with only the semiconductor.

Comparative Example 7

In the configuration of Example 11, the complex catalyst was notapplied, and the catalytic activity was measured with only thesemiconductor.

Comparative Example 8

In the configuration of Example 5, the working electrode was changed tothe glassy carbon electrode, and the catalytic activity was measured.

Comparative Example 9

In the configuration of Example 6, [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) was notused, and only [Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂](refer to FIG. 6( b)) was applied, and the catalytic activity wasmeasured.

Comparative Example 10

In the configuration of Example 13, the catalytic activity was measurednot under the carbon dioxide gas atmosphere, but under the argon gasatmosphere.

[Result]

TABLES 2 and 3 show the results of the catalytic activity measurementsof the above-described Examples 5-18 and Comparative Examples 4-10.

TABLE 2 APPLIED IRRADIATION HCOO⁻ SATURATED POTENTIAL PERIOD CONCEN- EFFCATHODE ANODE GAS SOLVENT (V; VS Ag/AgCl) (HOURS) TRATION (mM) (%)EXAMPLE 5 InP/Ru COMPLEX GC CO₂ DISTILLED −0.4 1 0.155 81.1 WATEREXAMPLE 6 InP/Ru COMPLEX GC CO₂ DISTILLED −0.4 1 0.197 78.2 WATERCOMPARATIVE InP GC CO₂ DISTILLED −0.6 3 0 0 EXAMPLE 4 WATER EXAMPLE 7GaP/Ru COMPLEX GC CO₂ DISTILLED −0.4 1 0.001 44.7 WATER EXAMPLE 8 GaP/RuCOMPLEX GC CO₂ DISTILLED −0.4 1 0.109 56.7 WATER COMPARATIVE GaP GC CO₂DISTILLED −0.4 1 0 0 EXAMPLE 5 WATER EXAMPLE 9 p-Si/Ru COMPLEX GC CO₂DISTILLED −0.4 1 0.006 52.6 WATER EXAMPLE 10 p-Si/Ru COMPLEX GC CO₂DISTILLED −0.4 1 0.018 75.5 WATER COMPARATIVE p-Si GC CO₂ DISTILLED −0.41 0 0 EXAMPLE 6 WATER EXAMPLE 11 N—Ta₂O₅/ GC CO₂ DISTILLED −0.4 1 0.02248.2 Ru COMPLEX WATER EXAMPLE 12 N—Ta₂O₅/ GC CO₂ DISTILLED −0.4 1 0.02963.3 Ru COMPLEX WATER COMPARATIVE N—Ta₂O₅/ GC CO₂ DISTILLED −0.4 1 0 0EXAMPLE 7 Ru COMPLEX WATER EXAMPLE 13 InP/Ru COMPLEX GC CO₂ NaHCO₃ −0.41 0.311 70.6 EXAMPLE 14 InP/Ru COMPLEX GC CO₂ Na₃PO₄ −0.4 1 0.201 58.6EXAMPLE 15 InP/Ru COMPLEX GC CO₂ Na₂SO₄ −0.4 1 0.243 58.1 COMPARATIVEInP GC CO₂ NaHCO₃ −0.4 1 0.001 5.7 EXAMPLE 10

TABLE 3 APPLIED HCOO⁻ MIXTURE SATURATED POTENTIAL CONCENTRATION EFFCATHODE RATIO ANODE GAS SOLVENT (V; VS Ag/AgCl) (mM) (%) EXAMPLE 6InP/Ru COMPLEX 1:1 GC CO₂ DISTILLED −0.4 0.197 78.2 WATER EXAMPLE 16InP/Ru COMPLEX 1:4 GC CO₂ DISTILLED −0.4 0.128 68.4 WATER EXAMPLE 17InP/Ru COMPLEX 4:1 GC CO₂ DISTILLED −0.4 0.163 80.8 WATER EXAMPLE 18InP/Ru COMPLEX 9:1 GC CO₂ DISTILLED −0.4 0.198 81.2 WATER COMPARATIVEInP/Ru COMPLEX 0:1 GC CO₂ DISTILLED −0.4 0.040 23.9 EXAMPLE 9 WATER

In the case of Comparative Example 4 in which the measurement wasconducted with only the semiconductor, as opposed to Example 5 in which0.155 mM of formic acid was detected in one hour under the carbondioxide gas atmosphere, only 0.01 mM of formic acid was detected inthree hours. In other words, reduction of carbon dioxide to the formicacid in the aqueous solution was suggested merely by applying a complexcatalyst on the indium phosphide electrode. In Example 6, a complexcatalyst having an anchor ligand and a complex catalyst having a pyrroleligand were combined, and, as a result, 0.197 mM of formic acid wasdetected in one hour, and the amount of production was increased ascompared to Example 5. This can be considered to have been caused by theimprovement of electron movement velocity between the semiconductor andthe complex catalyst and consequent improvement in the reactivity, dueto the use of the anchor ligand.

In Example 7, only 0.001 mM of formic acid was detected in one hourunder the carbon dioxide gas atmosphere, and, in Comparative Example 5in which the measurement was conducted only with the semiconductor, only0.001 mM of formic acid was detected in one hour. In Example 8, acomplex catalyst having an anchor ligand and a complex catalyst having apyrrole ligand were combined, and, as a result, 0.109 mM of formic acidwas detected in one hour, and the amount of production was increased ascompared to Example 7 and Comparative Example 5. This can also beconsidered to have been caused by the smoothening of the electronmovement between the semiconductor and the complex catalyst andconsequent improvement of reactivity, due to the use of the anchorligand. In addition, it was suggested that the reduction reaction fromcarbon dioxide to the formic acid in the aqueous solution was caused bymerely applying the complex catalyst on the gallium phosphide electrode.

In Example 9, 0.006 mM of formic acid was detected in one hour under thecarbon dioxide gas atmosphere, whereas in Comparative Example 6 in whichthe measurement was conducted only with the semiconductor, only 0.002 mMof formic acid was detected in one hour. In other words, reduction ofcarbon dioxide to the formic acid in the aqueous solution was suggestedmerely by applying the complex catalyst on the p-type silicon electrode.In Example 10, a complex catalyst having an anchor ligand and a complexcatalyst having a pyrrole ligand were combined, and, as a result, 0.018mM of formic acid was detected in one hour, and the amount of productionwas increased compared to Example 9. This can also be considered to havebeen caused by the improvement of the electron movement velocity betweenthe semiconductor and the complex catalyst and consequent improvement ofreactivity, due to the use of the anchor ligand.

In Example 11, 0.022 mM of formic acid was detected in one hour underthe carbon dioxide gas atmosphere, whereas in Comparative Example 7 inwhich the measurement was conducted only with the semiconductor, only0.01 mM of formic acid was detected in one hour. In other words,reduction of carbon dioxide to the formic acid in the aqueous solutionwas suggested by merely applying the complex catalyst on the N—Ta₂O₅electrode. In Example 12, a complex catalyst having an anchor ligand anda complex catalyst having a pyrrole ligand were combined, and, as aresult, 0.029 mM of formic acid was detected in one hour, and the amountof production was increased as compared to Example 11. This can also beconsidered to have been caused by the smoothening of the electronmovement between the semiconductor and the complex catalyst andconsequent improvement in reactivity, due to the use of the anchorligand.

In Comparative Example 8, no formic acid was detected even when avoltage of −0.6 V (vs silver/silver chloride electrode) was applied for20 hours on the glassy carbon electrode under the carbon dioxide gasatmosphere. Based on this, it can be deduced that in Examples 5-12,various semiconductor electrodes use the light energy to enableproduction of formic acid at a low voltage.

From Comparative Example 9 and Examples 5, 6, and 16-18, it was foundthat the catalyst activity was low when single structure of[Ru({4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂)] was used. Inaddition, it was found that the catalyst activity was improved by mixing[Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] and [Ru{4,4′-diphosphateethyl-2,2′-bipyridine}(CO)₂Cl₂], and the mixture ratio of 1:1 to 9:1results in a high effect on the activity improvement.

Upon comparison of Examples 13-15 and Comparative Example 10, it wasfound that the reduction reaction of carbon dioxide is not blocked evenwhen salt was added. In particular, it was found that the catalystactivity was improved when NaHCO₃ was added. In Comparative Example 10,it was found that formic acid was produced from NaHCO₃, and the catalystactivity is not improved.

A ruthenium complex [Ru(dpebpy)(bpy)(CO)₂]²⁺ (FIGS. 7 and 8) having a4,4′-diphosphate ethyl-2,2′-bipyridine ligand (dpebpy) was adsorbed onzinc-doped gallium phosphide (p-GaP—Zn manufactured by Sumitomo ElectricIndustries) in the following manner, and presence or absence of theadsorption of the ligand to the semiconductor substrate was analyzed.Specifically, 1 ml of dichloromethane/methanol mixture solution of 2 mMruthenium complex [Ru(dpebpy) (bpy) (CO)₂]²⁺ and a wafer (5 mm×5 mm) ofp-GaP—Zn were inserted into a container made of Teflon (registeredtrademark), left for one night at the room temperature and taken out,washed with a solvent (dichloromethane/methanol) twice, and vacuum driedat a temperature of 40° C. FIG. 9 shows a result of analysis of thissample by a time-of-flight secondary ion mass spectrometry (TOF-SIMS).

In addition, a ruthenium complex [Ru(dpebpy) (bpy) (CO)₂]²⁺ (FIGS. 7 and8) having a 4,4′-diphosphate ethyl-2,2′-bipyridine ligand (dpebpy) wasadsorbed on zinc-doped indium phosphide (p-InP—Zn manufactured bySumitomo Electric Industries) in the following manner, and the presenceor absence of adsorption of the ligand on the semiconductor substratewas analyzed. Specifically, 1 ml of dichloromethane/methane mixturesolution of 2 mM ruthenium complex [Ru(dpebpy) (bpy) (CO)₂]²⁺ and awafer (5 mm×5 mm) of p-InP—Zn were inserted in a container made ofTeflon (registered trademark), left for one night at room temperatureand taken out, washed with a solvent (dichloromethane/methanol) twice,and vacuum dried at a temperature of 40° C. FIG. 10 shows a result ofanalysis of this sample by the time-of-flight secondary ion massspectrometry (TOF-SIMS).

From FIGS. 9 and 10, it was found that a spectrum corresponding to theruthenium ion was observed and that the ruthenium complex was adsorbedon gallium phosphide and indium phosphide.

Next, in Example 16, the reduction reaction electrode 10 for reducingcarbon dioxide and the oxidation reaction electrode 12 for oxidizingwater and generating oxygen were combined, and a value of photocurrentwhen the bias voltage applied between the two electrodes was set to 0 Vwas checked.

Example 19

For the reduction reaction electrode 10, a wafer (8 mm×20 mm) ofzinc-doped indium phosphide (p-InP—Zn manufactured by Sumitomo ElectricIndustries), which is a p-type semiconductor was used, and, for theoxidation reaction electrode 12, TiO₂(P25) electrode formed on aconductive glass (FTO manufactured by Asahi Glass) by a squeegee methodusing commercially available titanium oxide (TiO₂) particles (P25manufactured by Degussa) was used. The TiO₂(P25) electrode includesapproximately 80% of anatase-type titanium oxide.

For the photoelectrochemical measurement, an electrochemical analyzer(BAS) was used, and the measurement was conducted in the two-electrodesystem which uses the working electrode and the counter electrode. Thereduction reaction electrode 10 was used for the working electrode, theoxidation reaction electrode 12 was used for the counter electrode, andthe two electrodes were placed in parallel and in an overlapping manner.A rectangular quartz glass cell was used for the cell, and 2.5 ml of 0.2M K₂SO₄ was used for the electrolyte. A xenon lamp of 300 W (MAX-302manufactured by Asahi Spectra) was used for the light source, appliedvoltage was set to 0 V, and light of all wavelengths was irradiated fromthe side of the oxidation reaction photoelectrode.

Comparative Example 11

A wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Znmanufactured by Sumitomo Electric Industries), which is a p-typesemiconductor, was used for the reduction reaction electrode 10, and aTiO_(2-x) electrode in which monocrystal of rutile-type titanium oxidewas reduced by hydrogen was used for the oxidation reaction electrode12. The other conditions were set to be the same as those of Example 19.

[Result]

TABLE 4 shows a result of photoelectrochemical measurement on Example 19and Comparative Example 11. In Example 19, with the use of the TiO₂(P25)electrode primarily including anatase-type titanium oxide, aphotocurrent of 21 μA was observed under the condition of the appliedvoltage of 0 V. On the other hand, in Example 11 in which TiO_(2-x)electrode which is a rutile-type titanium oxide was used, thephotocurrent at the applied voltage of 0 V was 4.4 μA. Therefore, thephotocurrent was increased by factor of four or greater with the use ofthe anatase-type titanium oxide.

TABLE 4 APPLIED POTENTIAL PHOTOCURRENT CATHODE ANODE SOLVENT (V; VSAg/AgCl) (μA) EXAMPLE 19 InP TiO₂ (P25) K₂SO₄ 0 21 COMPARATIVE InPTiO_(2−x) K₂SO₄ 0 4.4 EXAMPLE 11

An energy level of the conduction band of the anatase-type titaniumoxide is positioned on a negative side with respect to an energy levelof the conduction band of the rutile-type titanium oxide, and, with theuse of the anatase-type titanium oxide, a difference with respect to anenergy level of the valance band of the indium phosphide is larger ascompared to the structure where the rutile-type titanium oxide is used.In other words, it can be deduced that, with the use of the anatase-typetitanium oxide, a potential gradient created between the two electrodesis increased, movement of electrons between the electrodes is promoted,and the photocurrent was increased even with the applied voltage of 0 V.

Next, in Examples 20 and 21, the reduction reaction of carbon oxide whenthe reduction reaction electrode 10 and the oxidation reaction electrode12 were combined and water was used as the electron donor was checked.For the photoelectrochemical measurement, as shown in FIG. 11, anelectrochemical analyzer (BAS) was used, and the measurement wasconducted in the two-electrode system which uses the working electrodeand the counter electrode. For the cell, a two-chamber cell separated bya proton exchange membrane (Nafion 117 manufactured by Du Pont) wasused. For the light source, a xenon lamp of 300 W (MAX-302 manufacturedby Asahi Spectra) or a solar simulator (HAL-320 manufactured by AsahiSpectra) was used. For evaluation of the product involved with thephotoelectrochemical measurement, ion chromatograph (DIONEX withICS-2000 auto-sampler AS) was used. For the column, IonPac AS15 wasused. For the eluate, a KOH eluate was used, and, for the detector, anelectric conductivity detector was used.

Example 20

For the reduction reaction electrode 10, an MeCN solution containing[Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂].FeCl₃.pyrrol was applied on awafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Zn manufacturedby Sumitomo Electric Industries), which is a p-type semiconductor,dried, and then washed with water and used. For the oxidation reactionelectrode 12, a TiO₂ (P25) electrode created on a conductive glass (FTOmanufactured by Asahi Glass) by a squeegee method using commerciallyavailable titanium oxide (TiO₂) particles (P25 manufactured by Degussa)was used. For the electrolyte, 5 ml of distilled water was used. Afterargon gas was bubbled in the solution for about 20 minutes to remove thedissolved gas, carbon dioxide gas was bubbled in the solution for about10 minutes, and then, the measurement was conducted under carbon dioxidegas atmosphere. A bias voltage between the two electrodes was set to 0V, and xenon light corresponding to 1.4 SUN was irradiated.

Example 21

For the reduction reaction electrode 10, an MeCN solution containingFeCl₃.pyrrol in which [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] and [Ru({4,4′-diphosphateethyl-2,2′-bipyridine}(CO)₂Cl₂)] were mixed in a 1:1 ratio were appliedon a wafer (8 mm×20 mm) of zinc-doped indium phosphide (p-InP—Znmanufactured by Sumitomo Electric Industries), which is a p-typesemiconductor, dried, and then washed with water and used. For theoxidation reaction electrode 12, there was used an electrode in whichplatinum was carried on a TiO₂(P25) electrode created on a conductiveglass (FTO manufactured by Asahi Glass) by a squeegee method usingcommercially available titanium oxide (TiO₂) particles (P25 manufacturedby Degussa). For the electrolyte, 5 ml of an aqueous solution of 10 mMNaHCO₃ was used. After argon gas was bubbled in the solution for about20 minutes to remove the dissolved gas, carbon dioxide gas was bubbledin the solution for about 10 minutes, and the measurement was conductedunder the carbon dioxide gas atmosphere. A bias voltage between the twoelectrodes was set to 0 V, and light of a solar simulator correspondingto 1 SUN was irradiated.

[Result]

TABLE 5 shows results of photoelectrochemical measurements with respectto Examples 20 and 21. In Example 20, 0.115 mM of formic acid wasdetected in 20 hours under the carbon dioxide gas atmosphere, and, whena ratio of charges (EFF) consumed for production of formic acid comparedto the total amount of observed charges was calculated, the ratio wasfound to be 35.8%. In Example 21, 0.190 mM of formic acid was detectedin 3 hours under the carbon dioxide gas atmosphere, and, when a ratio ofcharges (EFF) consumed for production of formic acid compared to thetotal amount of observed charges was calculated, the ratio was found tobe 67.2%.

TABLE 5 APPLIED IRRADIATION HCOO⁻ SATURATED POTENTIAL PERIODCONCENTRATION EFF CATHODE ANODE GAS SOLVENT (V; VS Ag/AgCl) (HOURS) (mM)(%) EXAMPLE 20 InP/Ru TiO₂ (P25) CO₂ DISTILLED 0 20 0.115 35.8 COMPLEXWATER EXAMPLE 21 InP/Ru Pt/TiO₂ (P25) CO₂ NaHCO₃ 0 3 0.190 67.2 COMPLEX

The TiO₂ (P25) electrode contains the anatase-type titanium oxide. Anenergy level of the conduction band of the anatase-type titanium oxideis high, and an energy difference with the valance band of indiumphosphide (p-InP—Zn) is large. Therefore, it can be deduced thatelectrons efficiently move between the two electrodes even under thezero-bias condition, and a large photocurrent is generated. In addition,it can be considered that, with the use of the two-chamber cell, thereaction fields of the oxidation reaction and the reduction reaction areseparated, resulting in inhibition of re-oxidation of the formic acid,produced by the reduction reaction, into carbon dioxide by an oxidationreaction.

As described above, by combining the reduction reaction electrode 10 forreducing carbon dioxide and the photoelectrode for oxidizing water andgenerating oxygen, a reduction reaction of carbon dioxide using water asthe electron donor can be realized.

Next, in Examples 22-24, a sulfide semiconductor and a metal complexwere combined for the reduction reaction electrode 10, and the reductionreaction of carbon dioxide using water as the electron donor waschecked.

For photoelectrochemical measurement for Examples 22-24 and ComparativeExamples 12-14, the electrochemical analyzer (BAS) was used, andmeasurement was conducted in the three-electrode system which uses theworking electrode, the counter electrode, and the reference electrode ina structure as shown in FIG. 7. For the cell, a cylindrical Pyrex(registered trademark) glass cell was used. For the light source, axenon lamp of 300 W (MAX-302 manufactured by Asahi Spectra) was used,and, using a cutoff filter of a wavelength of 422 nm, only visible lightwas irradiated. For the evaluation of product involved with thephotoelectrochemical measurement, an ion chromatograph (DIONEX withICS-2000 auto-sampler AS) was used. For the column, IonPac AS15 wasused, for the eluate, a KOH eluate was used, and for the detector, anelectric conductivity detector was used.

Example 22

For the working electrode, there was used an electrode in whichgallium-doped Cu₂ZnSnS₄(Ga-CZTS), which is a p-type semiconductor, wasmodified with a ruthenium complex polymer. An MeCN solution containingFeCl₃.pyrrol in which ruthenium complex polymers[Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and[Ru({4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂)] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTS substrate, dried,and then washed with water and used. For the counter electrode, theglassy carbon electrode was used, and, for the reference electrode, thesilver/silver chloride electrode (Ag/AgCl) was used. For theelectrolyte, 5 ml of purified water was used. After argon gas wasbubbled in the solution for about 20 minutes to remove the dissolvedgas, carbon dioxide gas was bubbled in the solution for about 10minutes, and then, light of 70 SUN was irradiated under the carbondioxide gas atmosphere, and the reduction and oxidation reactions weremeasured. For the potential, −0.4 V was applied with respect to thereference electrode.

Example 23

For the working electrode, there was used an electrode in which Cu₂ZnSn(S, Se)₄ (CZTSSe) which is a p-type semiconductor was modified by aruthenium complex polymer. An MeCN solution containing FeCl₃.pyrrol inwhich ruthenium complex polymers [Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO) (MeCN) Cl₂] (refer to FIG. 6( a)) and[Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTSSe substrate, dried,and then washed with water and used. For the counter electrode, theglassy carbon electrode was used and, for the reference electrode, thesilver/silver chloride electrode (Ag/AgCl) was used. For theelectrolyte, 5 ml of purified water was used. After argon gas wasbubbled in the solution for about 20 minutes to remove the dissolvedgas, carbon dioxide gas was bubbled in the solution for about 10minutes, and then, light of 70 SUN was irradiated under the carbondioxide gas atmosphere and the reduction and oxidation reactions weremeasured. For the potential, −0.4 V was applied with respect to thereference electrode.

Example 24

For the working electrode, there was used an electrode in whichCu₂ZnSnS₄ (CZTS), which is a p-type semiconductor, was modified with aruthenium complex polymer. An MeCN solution containing FeCl₃.pyrrol inwhich ruthenium complex polymers [Ru {4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and[Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTS substrate, dried,and then washed with water and used. For the counter electrode, theglassy carbon electrode was used and, for the reference electrode, thesilver/silver chloride electrode (Ag/AgCl) was used. For theelectrolyte, 5 ml of purified water was used. After argon gas wasbubbled in the solution for about 20 minutes to remove the dissolvedgas, carbon dioxide gas was bubbled in the solution for about 10minutes, and then, light of 70 SUN was irradiated under the carbondioxide gas atmosphere and the reduction and oxidation reactions weremeasured. For the potential, −0.4 V was applied with respect to thereference electrode.

Comparative Example 12

For the working electrode, gallium-doped Cu₂ZnSnS₄ (Ga-CZTS), which is ap-type semiconductor, was used. For the counter electrode, the glassycarbon electrode was used and, for the reference electrode, thesilver/silver chloride electrode (Ag/AgCl) was used. For theelectrolyte, 5 ml of purified water was used. After argon gas wasbubbled in the solution for about 20 minutes to remove the dissolvedgas, carbon dioxide gas was bubbled in the solution for about 10minutes, and then, light of 70 SUN was irradiated under the carbondioxide gas atmosphere and the reduction and oxidation reactions weremeasured. For the potential, −0.4 V was applied with respect to thereference electrode.

Comparative Example 13

For the working electrode, there was used an electrode in whichgallium-doped Cu₂ZnSnS₄ (Ga-CZTS), which is a p-type semiconductor, wasmodified with a ruthenium complex polymer. An MeCN solution containingFeCl₃.pyrrol in which ruthenium complex polymers[Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and[Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTS substrate, dried,and then washed with water and used. For the counter electrode, theglassy carbon electrode was used and, for the reference electrode, thesilver/silver chloride electrode (Ag/AgCl) was used. For theelectrolyte, 5 ml of purified water was used. After argon gas wasbubbled in the solution for about 20 minutes to remove the dissolvedgas, light of 70 SUN was irradiated under an argon gas atmosphere, andthe reduction and oxidation reactions were measured. For the potential,−0.4 V was applied with respect to the reference electrode.

Comparative Example 14

For the working electrode, there was used an electrode in whichgallium-doped Cu₂ZnSnS₄ (Ga-CZTS), which is a p-type semiconductor, wasmodified with a ruthenium complex polymer. An MeCN solution containingFeCl₃.pyrrol in which ruthenium complex polymers[Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and [Ru{4,4′-diphosphateethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6( b)) were mixed in a1:1 ratio was applied on a CZTS substrate, dried, and then washed withwater and used. For the counter electrode, the glassy carbon electrodewas used and, for the reference electrode, the silver/silver chlorideelectrode (Ag/AgCl) was used. For the electrolyte, 5 ml of purifiedwater was used. After argon gas was bubbled in the solution for about 20minutes to remove the dissolved gas, carbon dioxide gas was bubbled inthe solution for about 10 minutes, and then, the reduction and oxidationreactions were measured under the carbon dioxide gas atmosphere and withradiation of no light. For the potential, −0.4 V was applied withrespect to the reference electrode.

[Result]

TABLE 6 shows results of the photoelectrochemical measurements forExamples 22-24 and Comparative Examples 12-14.

TABLE 6 APPLIED IRRADI- HCOO⁻ SATUR- POTENTIAL ATION CONCEN- ATED (V; VSIRRADIATED PERIOD TRATION EFF CATHODE ANODE GAS SOLVENT Ag/AgCl) LIGHT(HOURS) (mM) (%) EXAMPLE 22 Ga-CZTS/ GC CO₂ DISTILLED −0.4 70SUN (λ > 30.246 74 Ru COMPLEX WATER 422 nm) EXAMPLE 23 CZTSSe/ GC CO₂ DISTILLED−0.4 70SUN (λ > 3 0.382 71 Ru COMPLEX WATER 422 nm) EXAMPLE 24 CZTS/ GCCO₂ DISTILLED −0.6 70SUN (λ > 3 0.285 82 Ru COMPLEX WATER 422 nm)COMPARATIVE Ga-CZTS GC CO₂ DISTILLED −0.4 70SUN (λ > 3 0.006 — EXAMPLE12 WATER 422 nm) COMPARATIVE Ga-CZTS/ GC Ar DISTILLED −0.4 70SUN (λ > 30.008 — EXAMPLE 13 Ru COMPLEX WATER 422 nm) COMPARATIVE Ga-CZTS/ GC CO₂DISTILLED −0.4 DARK CONDITION 3 0 — EXAMPLE 14 Ru COMPLEX WATER

In Example 22, 0.246 mM of formic acid was detected when light wasirradiated for three hours under the carbon dioxide gas atmosphere, anda ratio of charges consumed for production of formic acid with respectto a total amount of measured charges was 74.1%. On the other hand, inComparative Example 12 in which light was irradiated only on thesemiconductor under the carbon dioxide gas atmosphere, only 0.006 mM offormic acid was detected in three hours. In Comparative Example 13 inwhich light was irradiated under the argon gas atmosphere, only 0.008 mMof formic acid was detected in three hours. In Comparative Example 14 inwhich light was not irradiated under the carbon dioxide gas atmosphere,no formic acid was detected. From the above-described results, it wassuggested that the Ga-CZTS electrode modified with the ruthenium complexpolymer has selectively reduced with light carbon dioxide into formicacid in an aqueous solution.

In Example 23 in which light was irradiated under the carbon dioxide gasatmosphere for three hours, 0.382 mM of formic acid was detected, and aratio of charges consumed for production of formic acid with respect toa total amount of measured charges was 71.2%. With the use of the CZTSSeelectrode also, similar to the CZTS electrode, carbon dioxide can beselectively reduced with light into formic acid.

In Example 24 in which light was irradiated under the carbon dioxide gasatmosphere for three hours, 0.285 mM of formic acid was detected, and aratio of charges consumed for production of the formic acid with respectto a total amount of measured charges was 81.8%. Based on themeasurement results of the current-voltage characteristic, the uppermostenergy level of the valance band of CZTS can be estimated to be at apotential of about 0.2 V with respect to the reference electrode(Ag/AgCl). In consideration of the bandgap of 1.5 eV, the lowermostenergy level of the conduction and is at a potential more negative thanthe potential of −0.6 V necessary for reducing carbon dioxide on theruthenium complex polymer. Because of this, it can be deduced that theelectrons excited to the conduction band can move to the complex, andthe reduction reaction of carbon dioxide has progressed on the complex.In CZTSSe also, it can be deduced that the reaction has occurred with asimilar mechanism.

The above-described reduction reaction electrode (compositephotoelectrode) for reducing carbon dioxide and the oxidation reactionelectrode (photoelectrode) for oxidizing water and generating oxygenwere combined and a reduction reaction of carbon dioxide using water asthe electron donor was measured. For the photoelectrochemicalmeasurement, the electrochemical analyzer (BAS) was used, and themeasurement was conducted in the two-electrode system which uses theworking electrode and the counter electrode. For the cell, a two-chambercell separated by a proton exchange membrane (Nafion 117 manufactured byDu Pont) was employed. For the light source, a solar simulator (HAL-320manufactured by Asahi Spectra) was used. For evaluation of productsinvolved with the photoelectrochemical measurement, ion chromatograph(DIONEX with ICS-2000 auto-sampler AS) was used. For the column, IonPacAs15 was used, for the eluent, KOH eluent was used, and for thedetector, an electric conductivity detector was used.

Example 25

For the working electrode, there was used an electrode in whichCu₂ZnSn(S,Se)₄ (CZTSSe), which is a p-type semiconductor, was modifiedwith a ruthenium complex polymer. An MeCN solution containingFeCl₃.pyrrol in which ruthenium complex polymers[Ru{4,4′-di(1-H-1-pyrrolypropylcarbonate)-2,2′-bipyridine}(CO)(MeCN)Cl₂] (refer to FIG. 6( a)) and[Ru{4,4′-diphosphate ethyl-2,2′-bipyridine}(CO)₂Cl₂] (refer to FIG. 6(b)) were mixed in a 1:1 ratio was applied on a CZTSSe substrate, dried,and then washed with water and used. For the counter electrode, therewas used an electrode in which titanium oxide (TiO₂) particles (P25manufactured by Degussa) were applied on a conductive glass (FTOmanufactured by Asahi Glass) through a squeegee method and platinum wascarried on the titanium oxide (TiO₂) electrode. For the electrolyte, 4ml of an aqueous solution of 10 mM NaHCO₃ was used in each cell. Afterargon gas was bubbled for about 20 minutes in the solution to remove thedissolved gas, carbon dioxide gas was bubbled in the solution for about10 minutes, and then, the reduction and oxidation reactions weremeasured under the carbon dioxide gas atmosphere. No bias voltage wasapplied (0 V), and light of solar simulator corresponding to 1 SUN wasirradiated.

[Result]

TABLE 7 shows a result of the photoelectrochemical measurements forExample 25 and Comparative Examples 12-14.

TABLE 7 APPLIED IRRADI- IRRADIATION HCOO⁻ SATURATED POTENTIAL ATEDPERIOD CONCEN- EFF CATHODE ANODE GAS SOLVENT (V) LIGHT (HOURS) TRATION(mM) (%) EXAMPLE 25 CZTSSe/Ru Pt/TiO₂(P25) CO₂ NaHCO₃ 0 1SUN 3 0.1 52COMPLEX

In Example 25 in which light was irradiated under the carbon dioxide gasatmosphere for three hours, 0.1 mM of formic acid was detected, and aratio of charges consumed for production of formic acid with respect toa total amount of measured charges was 51.7%. It can be considered that,because titanium oxide (TiO₂) electrode (P25) contains anatase-typetitanium oxide, the energy level of the conduction band of theanatase-type titanium oxide is high, and an energy difference with thevalance band of the CZTSSe electrode is large, the electrons efficientlymove between the two electrodes and a photocurrent was generated evenunder the condition that no bias voltage was applied (0 V).

EXPLANATION OF REFERENCE NUMERALS

-   10 REDUCTION REACTION ELECTRODE; 12 OXIDATION REACTION ELECTRODE; 14    BIAS POWER SUPPLY; 16 BASE MATERIAL

1. A photochemical reaction device comprising: an oxidation reactionelectrode which oxidizes water and generates oxygen; and a reductionreaction electrode which reduces carbon dioxide and synthesizes a carboncompound, wherein the oxidation reaction electrode and the reductionreaction electrode are electrically connected, and the reductionreaction electrode reduces carbon dioxide and synthesizes the carboncompound in a solution containing water, by means of energy ofirradiated light.
 2. The photochemical reaction device according toclaim 1, wherein an energy level of a conduction band minimum of theoxidation reaction electrode is positioned at a potential on a negativeside with respect to an energy level of a valance band minimum of thereduction reaction electrode.
 3. The photochemical reaction deviceaccording to claim 1, wherein the reduction reaction electrode has astructure in which a semiconductor electrode and a catalyst whichpresents a reduction action of carbon dioxide are coupled, and thereduction reaction of carbon dioxide is presented by movement of excitedelectrons generated by radiation of light on the semiconductor electrodeto the catalyst.
 4. The photochemical reaction device according to claim1, wherein the reduction reaction electrode has a structure in which asemiconductor electrode and a catalyst which presents a reduction actionof carbon dioxide are coupled by chemical polymerization, and thereduction reaction electrode reduces carbon dioxide and synthesizes thecarbon compound in the solution containing water by means of the energyof irradiated light.
 5. The photochemical reaction device according toclaim 1, wherein the oxidation reaction electrode and the reductionreaction electrode are placed in two chambers separated by a protonexchange membrane, and the oxidation reaction electrode and thereduction reaction electrode are electrically connected, and thereduction reaction electrode reduces carbon dioxide and synthesizes thecarbon compound in the solution containing water by means of the energyof irradiated light.
 6. The photochemical reaction device according toclaim 1, wherein the oxidation reaction electrode and the reductionreaction electrode are electrically connected, the oxidation reactionelectrode is a semiconductor electrode, and oxidizes water and extractselectrons by means of the energy of irradiated light, and the reductionreaction electrode reduces carbon dioxide and synthesizes the carboncompound in the solution containing water by means of the energy ofirradiated light.
 7. The photochemical reaction device according toclaim 3, wherein the catalyst is a metal complex or a polymer thereof.8. The photochemical reaction device according to claim 7, wherein thecatalyst is a mixture of a first metal complex having an anchor sitewhich is connected to the semiconductor electrode and a second metalcomplex which is polymerized with the first metal complex and which hasa catalytic function.
 9. The photochemical reaction device according toclaim 8, wherein the second metal complex has a pyrrole site.
 10. Thephotochemical reaction device according to claim 8, wherein a chemicalpolymerization film of the first metal complex and the second metalcomplex is formed on a surface of the semiconductor electrode.
 11. Thephotochemical reaction device according to claim 1, wherein theoxidation reaction electrode and the reduction reaction electrode aredirectly connected in a state where no external bias voltage is applied,and light is irradiated on both electrodes so that water functions as anelectron donor for CO₂ reduction.
 12. The photochemical reaction deviceaccording to claim 1, wherein the oxidation reaction electrode and thereduction reaction electrode are connected in a state where a bias powersupply is applied, and light is irradiated on both electrodes so thatwater functions as an electron donor.
 13. The photochemical reactiondevice according to claim 1, wherein the oxidation reaction electrodecomprises titanium oxide.
 14. The photochemical reaction deviceaccording to claim 13, wherein the oxidation reaction electrodecomprises anatase-type titanium oxide.
 15. The photochemical reactiondevice according to claim 1, wherein the solution containing water iswater or an aqueous solution containing an electrolyte.
 16. Thephotochemical reaction device according to claim 1, wherein theoxidation reaction electrode and the reduction reaction electrode areseparated by an ion exchange membrane.
 17. The photochemical reactiondevice according to claim 1, wherein a three-electrode system structureis employed which has a reference electrode in addition to the oxidationreaction electrode and the reduction reaction electrode.
 18. A compositephotoelectrode comprising: a catalyst which presents a reductionreaction of carbon dioxide, and a semiconductor electrode coupled withthe catalyst, wherein the reduction reaction of carbon dioxide ispresented by transfer to the catalyst of excited electrons generated byradiation of light on the semiconductor electrode.
 19. The compositephotoelectrode according to claim 18, wherein the catalyst is a metalcomplex or a polymer thereof.
 20. The composite photoelectrode accordingto claim 18, wherein the semiconductor electrode is a sulfidesemiconductor or a phosphide semiconductor.
 21. A light energy storagedevice in which the composite photoelectrode according to claim 18 andan oxidation reaction electrode which oxidizes water and generatesoxygen are connected.